There is lower berth quota which is for the following categories of passengers when travelling alone:
a) Sr. Citizens;
b) Female passengers 45 years of age and above;
c) Pregnant Women.
So far as...
more... I know berths of different categories becoming vacant upon cancellation of cnf tickets are so allotted that compact accommodation can be provided for passengers under the same pnr. And I also think that the above categories of passengers get preference when it comes to allotting berths which have become vacant following cancellation of cnf tickets.